106 WAVES OF SAND AND SNOW 



during the previous day and night. By lo a.m. 

 this was already drifting in low waves which occu- 

 pied the whole prairie. I measured a series of 

 twenty-nine which had an average wave-length of 

 30 feet I inch and an average height of 72 inches, 

 the wave-length being therefore fifty times as great 

 as the height. The average difference between 

 the length of each wave and of that preceding it 

 was 22 per cent, of the mean wave-length. This 

 percentage I term' the variability of the wave- 

 length. Its record gives a measure of the degree 

 to which groups of waves depart from' exact 

 uniformity . 



Later in the morning, when drifting had con- 

 tinued longer, I measured a series of 1 1 o 

 consecutive waves which had an average wave- 

 length of 32 feet 10 inches. The variability of 

 wave-length was 36- 5 per cent. ; i.e., they were 

 less regular than those measured earlier. The 

 reason for this deterioration was easily detected. 

 As the waves rolled on, layers of snow in which 

 pressure had made the particles adhere came to. 

 the surface, and this material was not sufficiently 

 fluid to travel in waves. During the remainder of 

 this day and in the course of the two days follow- 

 ing, during which the wind blew but snow did not 

 fall, the appearance of the snow -covered prairie 



